SR850 Basics

INTRINSIC (RANDOM) NOISE SOURCES

Random noise finds its way into experiments in a variety of ways. Good experimental design can reduce these noise sources and improve the measurement stability and accuracy.

There are a variety of intrinsic noise sources which are present in all electronic signals. These sources are physical in origin.

Johnson noise

Every resistor generates a noise voltage across its terminals due to thermal fluctuations in the elec- tron density within the resistor itself. These fluctua- tions give rise to an open-circuit noise voltage,

1/2

V noise (rms) = ( 4k TRΔ f)

k-23

where =Boltzmann's constant (1.38x10 J/°K), T is the temperature in °Kelvin (typically 300°K), R is the resistance in Ohms, and Δf is the bandwidth in Hz. Δf is the bandwidth of the measurement.

Since the input signal amplifier in the SR850 has a bandwidth of approximately 300 kHz, the effective

noise at the amplifier input is Vnoise = 70R nVrms or 350R nV pk-pk. This noise is broadband and if

the source impedance of the signal is large, can determine the amount of dynamic reserve required.

The amount of noise measured by the lock-in is determined by the measurement bandwidth. Remember, the lock-in does not narrow its detec- tion bandwidth until after the phase sensitive detectors. In a lock-in, the equivalent noise band- width (ENBW) of the low pass filter (time constant) sets the detection bandwidth. In this case, the measured noise of a resistor at the lock-in input, typically the source impedance of the signal, is simply

V noise (rms) = 0.13 R ENBW nV

The SR850 displays the ENBW in the TIME CONSTANT menu. This is the correct noise band- width for the time constant and the number of poles and should be used to calculate the detect- ed Johnson noise. The displayed ENBW does not take the synchronous filter into account.

The signal amplifier bandwidth determines the

amount of broadband noise that will be amplified. This affects the dynamic reserve. The time con- stant sets the amount of noise which will be meas- ured at the reference frequency. See the SIGNAL INPUT AMPLIFIER discussion for more informa- tion about Johnson noise.

Shot noise

Electric current has noise due to the finite nature of the charge carriers. There is always some non- uniformity in the electron flow which generates noise in the current. This noise is called shot noise. This can appear as voltage noise when cur- rent is passed through a resistor, or as noise in a current measurement. The shot noise or current noise is given by

1/2

Inoise (rms) = (2q IΔ f)

where q is the electron charge (1.6x10-19Coulomb), I is the RMS AC current or DC current depending upon the circuit, and Δf is the bandwidth.

When the current input of a lock-in is used to measure an AC signal current, the bandwidth is typically so small that shot noise is not important.

1/f noise

Every 10 Ω resistor, no matter what it is made of, has the same Johnson noise. However, there is excess noise in addition to Johnson noise which arises from fluctuations in resistance due to the current flowing through the resistor. For carbon composition resistors, this is typically 0.1 µV-3 µV of rms noise per Volt of applied across the resis- tor. Metal film and wire-wound resistors have about 10 times less noise. This noise has a 1/f spectrum and makes measurements at low fre- quencies more difficult.

Other sources of 1/f noise include noise found in vacuum tubes and semiconductors.

Total noise

All of these noise sources are incoherent. The total random noise is the square root of the sum of the squares of all the incoherent noise sources.

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SRS Labs SR850 manual Intrinsic Random Noise Sources, Johnson noise, Shot noise, Total noise

SR850 specifications

The SRS Labs SR850 is a high-performance audio processor designed to enhance the listening experience across a variety of applications. With its advanced technologies, the SR850 delivers superior sound quality that is particularly noticeable in environments where audio clarity and fidelity are paramount. This device caters to audio professionals, audiophiles, and casual listeners who demand exceptional performance from their audio systems.

One of the standout features of the SR850 is its proprietary SRS 3D Audio technology. This innovative feature creates a more immersive soundstage, allowing users to experience audio as if they were in a live performance setting. By adding depth and dimension, it elevates the listening experience, making music, movies, and games far more engaging.

Additionally, the SR850 incorporates SRS dialog clarity technology, which enhances speech intelligibility in dialogues and vocals. This is particularly beneficial in film and television viewing, where dialogue can sometimes be drowned out by background music or sound effects. With dialog clarity, users can enjoy crystal-clear conversation tracks without adjusting volume levels continuously.

The device also supports various audio formats, ensuring compatibility with a broad range of media. It boasts inputs for both digital and analog sources, allowing connections to various devices such as TVs, laptops, gaming consoles, and more. The versatility of inputs makes the SR850 a convenient addition to any home entertainment system.

Another impressive characteristic of the SR850 is its compact and user-friendly design. The intuitive interface makes it easy for users to adjust settings to their preference, whether they are a novice or an audio expert. The device is equipped with programmable presets, enabling users to store their favorite sound profiles, making switching between settings quick and hassle-free.

Furthermore, the SR850 features advanced EQ capabilities, granting users the ability to tailor the audio output to their specific tastes. This personalized approach to sound adjustment enhances the overall experience, allowing users to get the most out of their audio content.

In summary, the SRS Labs SR850 is a sophisticated audio processor packed with features designed to significantly uplift sound quality and listening pleasure. With its cutting-edge technologies such as SRS 3D Audio and dialog clarity, an array of connectivity options, and customizable EQ settings, the SR850 stands out as an outstanding option for anyone looking to enhance their audio experience. Whether for professional use in sound engineering or personal enjoyment in home entertainment, the SR850 proves to be a valuable asset.